If you have a set of 0.250-0.500 pin gauges, you can find the one that just makes it down the inside of a case and see where it stops. It takes a bit more feel, but you can do something similar with a
half-ball small hole gauge by setting it to just barely rub inside the case mouth and then seeing how far down it goes before starting to jam. That will be where the case wall is thickening, and it limits your seating depth.
One of our members pointed out that some makes have or used to have different 38 Special brass for wadcutter target loads that stay thin in the wall to a greater depth before thickening. It's something I had been unaware of. I assume the cases would be marked "match" or have a different case cannelure location or some other indicator. Such a case would let you seat deeply, but would also be less strong, so I don't know whether they would be compatible with 9 mm pressure loads or not. You'd have to work up carefully while watching for signs of problems with them, to see. I am mainly mentioning this so you can see if sorting your brass by depth of the thin part of your wall might be useful. I would assume such cases would be lighter than their same-headstamp regular cases, too, giving you another way to sort.
In your shoes, I would be inclined to buy a small number of the Starline and compare to the .38 Special. Starline won't sell you less than 500 directly, but Midway repacks them in quantities of 100 so you could buy those without investing quite so much in the project. It might be a good way to try the concept out and look at them in speed loaders and other factors.
